Toning can be found on all series of U.S. gold, from the very first Turban Head gold coins onward. Unfortunately, due to the high rate of dipping, it is often difficult to find old gold coins with original deep-golden toning. Many 18th and 19th century coins are unnaturally bright for their age, visible evidence of dipping and potentially cleaning. The more dramatic red and purple advanced-stage toning is even scarcer, but does seem to be slightly more prevalent on Indian Head quarter and half-eagles. It is possible that the higher number of toned Indian Head gold coins (not including the $10 eagle) is related to the particular treatment that planchets for those two series of coins received. As the first U.S. coins with an incuse design, the quarter and half-eagles proved to be difficult to strike at each of the Mint facilities where they were produced (Philadelphia, Denver, and San Francisco). Eventually, one of the mint superintendents devised a solution of having the planchets shaved slightly before striking. This relieved some stress on the coining press, allowing the design to be struck with full details. My hypothesis is that the unique treatment of Indian Head planchets somehow made the coins more prone to toning - it is strictly conjecture, though.